Demystifying the Coordinate system : When is it applied in Civil 3D or Map ?

[cyberflow]

Hi there,

I know how to assign coordinate systems to my drawing, no problem.

But i'm trying to figure out when does Civil 3D work with coordinate ?

Is it when we create points when importing a survey ? When draw alignments ?

Or is it that we always work in a plan grid and then when we want to export there's a transformation applied ?

Thanx all in advance !
It's a pretty hot topic sometimes that is configured by a surveyor on a projet and we stick with that CS (coordinate system) for the life of the project without questionning ourselves.

[MichaelH13]

It works in co-ordinates from the moment a co-ordinate system is applied to it. Nothing mystifying about it.

[cyberflow]

Thx @MichaelH13

So if i draw 2 polylines in its seperate DWGs but assigne different coordinate systems then Xrefs them into another drawing with another coordinate system the 2 lines shouldnt be perfect at the same position ?

[BrianHailey]

The coordinate system comes into play when certain commands bring linework/data into or out of the drawing. The drawing is always a cartesian coordinate system. When you import things that have different coordinate systems applied to them (using certain commands), the transformation will be applied. When you export things out to a different coordinate system, again the transformation will be applied. 

The primary areas that coordinate systems are applied is when working with points, survey data, and GIS data. And even then, it may not be applied with every command in these areas.

[MichaelH13]

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@cyberflow wrote:

Thx @MichaelH13

So if i draw 2 polylines in its seperate DWGs but assigne different coordinate systems then Xrefs them into another drawing with another coordinate system the 2 lines shouldnt be perfect at the same position ?

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If you XREF a drawing into another with a different coordinate system, then your line will be in a different place.

Imagine where you are right now. In 1 coordinate system, your location will have a set of coordinate values. In another coordinate system, you location will have a different set of values. If you try and find your location in the 2nd coordinate system using the numbers from the first, you will be wildly out. 

This is the coordinate system I am working in:

The values on the right are the cursor coordinates for that coordinate system. Every object you place in your model will have coordinates assigned.

This polyline has coordinates for its vertexes:

Does this help?

[cyberflow]

Thx for the input @MichaelH13  & @BrianHailey 

And i'm wondering, i did some reading and the basic autocad objects can be transformed (lines, points)

But does Civil 3D objects get transformed too (Surfaces, Alignments, etc) ?

[Pointdump]

Frank,
Whole nuther ball of wax when you need to transform Civil 3D Objects. For that you need the >>>Coordinate Transformation Tool<<<.
Dave

[TerryDotson]

While the tool that @Pointdump mentioned does process many (not all) of the AECC objects, some don't lend themselves to being projected easily.  For example, TinSurfaces could be projected well but GridSurface (DEMs/TIFFs) as well as IMAGEs can't because they are rectangular and cannot become deformed into a Trapezoid, etc.  Also, I don't think the tool that he mentioned will allow projection between feet <> meter units.

Another thing to watch for when projected is alignments (and even polylines with curves).  The subtle difference in endpoint coordinates can throw out true curve tangencies.

[cyberflow]

Another thing i noticed ... Inroads / PowerInroads / Openroads Designer let's you draw geometries alignments, etc with a CSF (Combined scale factor) .....
For example a tagente with 100meters length could be after the GSF applied be 99.970m lets say

I never digged into that but is that something possible with C3D ?

I've always thought that C3D was always in grid when working and then we export things outside of the file.

[cyberflow]

Btw folks, i appreciate you coming and answering onto this post !

[cyberflow]

I did some searching and my conclusion on this is that it's impossible to work in Geodetic so C3D doesn't function like Inroads / PowerInroads / Openroads Designer does.

Everytime we work in C3D it's in cartesian plane (flat grid) and then we can export/transform to fit the geodetic distances.

[cyberflow]

Hi @Pointdump 

I tried the transformation tool ...

To my understanding, that only transforms :
Alignements, Profiles, Sample lines 

Am i correct ? 

And for the vertical transformation i don't need to primarely assign anything in vertical because C3D doesn't have any ways of assigning vertical datum

[Pointdump]

Hi Frank,
In theory the Transformation Tool transforms everything. Vertical Datums too. After it does its magic it gives you a report showing exactly what got transformed. To be honest, I haven't used it much, just a couple of test runs. It appears to be broken in 2025, so waiting for a fix.
Dave

[cyberflow]

In the french version of C3D it has an extra backslash "\" in the path of the vertical transformation files and makes the tool unusable : 

My other question was : 
How can i make that vertical transform between a NAVD88 and a CGVD2013 ?

I only see ellipsoid to EGM96 etc ...

[cyberflow]

And this makes me think of another thing.

In the drawing parameters the Transformation tab, what is it's use for ?

Those parameters are only used when importing something ?
If so what (Cogo, etc) ?

[cyberflow]

Ok so the transform tab is really only for import point :
https://www.youtube.com/watch?v=jJpNc3ZwtKE

[cyberflow]

I managed to make the transformation work with the Toolbox

But was always having crash messages with the profiles.

But ok one last one for the day : 
What are "Geo Entity" ? 

[Pointdump]

Frank,
"vertical transform between a NAVD88 and a CGVD2013"
I don't think those 2 vertical datums overlap, only maybe a little at the US/Canada border.
"the transform tab is really only for import point"
No, it transforms linework too. The Transformation Tab usually breaks Bing Live Maps, but it works well for most everything else.
"What are 'Geo Entity'?"
Dunno. Good question. Shapefiles?
Dave

[cyberflow]

Hi @Pointdump 

"vertical transform between a NAVD88 and a CGVD2013"
Your right, those 2 datums aren't the best example for my question.

My question was more, what if someone needs to transform to 2 different vertical datums that aren't in the list ..
I guess the person needs to create a bin file that is store :
C:\ProgramData\Autodesk\C3D 2024\enu\Geoid Models

Is it an easy thing to create those .bin files ?
They arent ASCII files so i guess only ADSK can create those

I've found this - With several Geoid Model files with .byn extension instead of .bin, wonder if it works : 
https://helpdesk.microsurvey.com/article/481-world-geoid-models-download

This helps clarify the difference between .byn and .bin files : 
https://natural-resources.canada.ca/sites/www.nrcan.gc.ca/files/earthsciences/pdf/gpshgrid_e.pdf

"What are 'Geo Entity'?"
For the Geo Entity, i did a comparison with all my object in the drawing and "Geo Entity" refers to "Network Part Connector" :

"the transform tab is really only for import point"
For the Transformation Tab, do you have an example of where it applies in linework ?

[Pointdump]

Hi Frank,
"vertical datums that aren't in the list..."
Somewhere in the Transformation Tool they used to show a website that stored all available Vertical Datum files. Binary files, I think, but I can't find it now.
Dave